The Internet Protocol originated as a communications protocol for a small group of trusted peers. Since all peers on the network were a priory trusted, the Internet Protocol and its two protocol suites, TCP/IP and UDP, do not contain any provisions for transmission authentication or protection against eavesdropping.
The TCP/IP protocol is the most popular network protocol of the Internet. Its popularity is due mostly to TCP/IP's utilization by the http protocol—the transport medium of the World Wide Web. Typically, TCP/IP transmission security is achieved in the higher levels of the OSI protocol stack (e.g., the SSL/TLS protocol at the application transport level) or by tunneling a TCP/IP communications session over a special IP level protocol (e.g., the IPSec protocol).
Since the TCP/IP protocol does not provide any capabilities that selectively permit or deny access to a TCP/IP server, any entity on the Internet is free to establish a connection to such server. A TCP/IP connection between two peers is established via a three-way handshake sub-protocol. During this interaction, communicating parties establish parameters of the TCP session to follow.
In order to initiate a new TCP/IP session, a peer who initiates the session (e.g., the client) transmits a special SYN TCP control packet to the target peer (e.g., the server). The SYN packet tells the server to synchronize packet exchange sequence numbers and contains the initialization parameter of a new work session, referred to as the client sequence number. The server responds to the client with another special control packet, called SYN/ACK, which acknowledges receipt of the session initiation packet and establishes the server side parameter of the TCP/IP session, referred to as the server sequence number. Upon receipt of the SYN/ACK packet, the client completes the TCP session establishment by transmitting the ACK control packet to the server, thus acknowledging the validity of the server sequence number established in the SYN/ACK packet. Once this connection establishment phase is complete, the peers may start exchanging data. A TCP/IP session is terminated when both peers transmit a control FIN packet and both sides acknowledge session termination by responding with a control ACK packet.
While being a necessary prerequisite to establishing a work session, this three way stateful initiation of the TCP/IP session allows malicious parties to determine the presence of a TCP/IP server on the network and to stage attacks against them. The first phase of a network-based attack is called “scanning” and consists of sending a SYN, a FIN, an ACK or a data packet to a suspected network location of a TCP/IP server, and then observing or not observing a return ACK or RST packet. Once the server is discovered, it is “fingerprinted”, i.e. the attacker identifies the type of software that provides the service. Once the type of software is determined, the attacker may use freely available tools to subvert, infiltrate, or crash the server. The assailant can also stage a special type of attack, called “Denial of Service” (DoS) attack, which floods a server with unanswered connection requests, e.g., a SYN Flood.